1. Field of the Invention
This invention relates to a mobile communication system in which a base station and mobile stations can communicate with each other by radio.
2. Description of the Prior Art
According to the Japanese automobile telephone system using digital radio communication, digital modulation is of the .pi./4-shift QPSK (quadriphase shift keying) type, and a plurality of carriers are used. During the radio communication, each of the carriers is subjected to 3-channel TDM (time division multiplexing).
In Europe, a GSM (global system for mobile communications) system using digital radio communication has been studied for practical use. According to the GSM system, digital modulation is of the GMSK (Gaussian filtered minimum shift keying) type, and a plurality of carriers are used. During the radio communication, each of the carriers is subjected to 8-channel TDM.
Furthermore, in Europe, a DECT (digital European cordless telecommunications) system has been investigated for private radio data transmission and private radio speech transmission within a small zone. According to the DECT system, digital modulation is of the GMSK type, and a plurality of carriers are used. In addition, TDM/TDD (time division multiplexing/time division duplex) is used in which equal numbers (about 10 channels) of uplinks and downlinks are assigned to each of the carriers.
An investigation has also been given of other various communication systems in which a base station and terminal stations in a service area covered by the base station can execute speech or data communication with each other by radio in time division multiplexing.
Time division multiplexing is used because of its advantages over frequency division multiplexing which will be described in the following. The first advantage is that a transmission data rate is variable and thus a certain degree of the flexibility of communication channels can be provided. The second advantage is that radio communication between the base station and the terminal stations has a burst shape in time base (time axis) and hence it is possible to set an interval of time during which operation is unnecessary.
On the other hand, time division multiplexing has the following disadvantages. As the degree of multiplexing increases, a transmission frequency band required for radio communication widens so that an equivalent noise bandwidth at a receiving side increases. Consequently, with respect to equal received signal levels, a received signal quality drops. In addition, with respect to equal transmission powers, an effective service area narrows. Further, radio communication in the present service area tends to be subject to interference from terminal stations located in neighboring service areas.
The other disadvantage of time division multiplexing will be described hereinafter. With respect to maintaining the qualities of received signals in communication channels resulting from time division, communication channels of equal times, equal modulation types, and a same carrier tend to interfere with each other. In addition, the communication channels tend to be subject to interference from another narrow-band system.
To maintain an adequate quality of a received signal of each communication channel, it is necessary that the received level of a desired signal is greater than that of an interfering signal by about 10 dB or more.
Also, a CDMA (code division multiple access) system has been investigated. In the CDMA system, information bits to be transmitted are spread in spectrum so as to be multiplexed within a common carrier according to spread code patterns having ten bits to several thousands of bits. The degree of multiplexing is determined by the number of different spread code patterns.
The CDMA system has the following advantages. Communication is hardly affected by narrow-band interference, in addition, many communication channels can be provided within a common carrier.
It is generally difficult for the previously-mentioned prior art communication systems to simultaneously satisfy enabling communications with different data rates, maintaining acceptable fiexibilities of communication channels, a great resistance to narrow-band interference, and permitting a common carrier to be used by many users.
The prior art spread spectrum communication system using the spread code patterns has the following problem. In the case where the received level of a signal of a desired communication channel which is spread in spectrum according to a spread code is extremely lower than the received levels of other signals spread in spectrum with other spread codes, a received signal quality of the desired communication channel tends to be lowered.